The invention is based on a hydraulic brake system for vehicles as defined hereinafter. A hydraulic brake system for vehicles, of this type and intended for preventing the vehicle wheels from locking, is described in a publication by Sumitomo Electric Industries Limited, Automotive Division, Japan. It cannot be precluded that the servo pressure source of such a vehicle brake system may suddenly fail, and that the valves that are intended to control the at least one brake pressure modulator may not close tightly, for instance because of contamination after an improperly performed servicing or by wearing of rubber seals from friction. In such a case, during braking the piston that is displaceable in the brake pressure modulator is no longer adequately supported by servo pressure fluid and can therefore yield to the brake pressure. This causes the brake pedal to sag downward, with the danger that the desired brake pressure and thus the desired deceleration of braking may no longer be brought about. If a plurality of brake pressure modulators are provided, then all of the pistons associated with them may disadvantageously yield, because if soil gets into the servo pressure source, it can spread to all of the valves.